Cindy De Jonge, Nathalie Dubois, Nemiah Ladd, Longhui Deng, Niroshan Gajendra, Negar Haghipour, Carsten Schubert, Mark Lever

To assess the long-term impact of climate change on mountain lakes and their sedimentary carbon storage, paleo-environmental approaches using well-dated lake sediment cores can be employed. For Rotsee, a perialpine lake near the Swiss Alps, a new sediment core has allowed the reconstruction of carbon mass accumulation rates for organic and inorganic carbon since 13ka BP. A multiproxy approach (XRF, carbon and nitrogen isotopes, organic macromolecule chemical compositions, and aDNA) was used to explore changes in the lake system that affect sedimentary carbon storage. Early Holocene warming led to a stepwise increase in inorganic carbon deposition, while organic carbon accumulation gradually increased throughout this period. Despite apparently affecting phytoplankton community proxies, warming during the Holocene Thermal Maximum did not directly impact sedimentary carbon storage. In the mid-Holocene, gradual lake infill caused shallowing and changes in water chemistry, redox conditions, and sources of organic matter. During this period, the sedimentary record indicates low pH, anoxic waters, and an influx of allochthonous, plant-derived organic matter, which increased organic carbon storage while inorganic carbon became negligible. In the Late Holocene, climatic or land use changes deepened the lake, leading to a more oxidized water column and a decline in allochthonous organic matter input. Still, larger deforestation events, such as during Roman times, coincided with increased organic carbon storage. Recent sediments, influenced by eutrophication in the last century, show intermediate carbon accumulation rates compared to earlier Holocene periods. Rotsee serves as a case study of how climate warming has influenced lake development and sedimentary carbon storage, with broader implications for understanding carbon dynamics in high-alpine lakes and their future carbon balance.